1. Field of the Invention
The present invention relates to devices for the production of sound and in particular to the production of sound of low frequencies in underwater applications.
2. Discussion of the Prior Art
Considerable effort has been expended in reducing the operating frequency of transducers that are used in underwater communication, sonar and other instrumentation. Whereas it is relatively straightforward to measure and record low frequency sound, such sound generation is more difficult. The current generation of low frequency transducers is generally based on piezoelectric or magnetostrictive materials operating within a flextensional capsule which increases the displacement of water produced by the device and hence renders its operation more efficient. However these transducers suffer from the disadvantage that to retain a given degree of efficiency in operation, their size must increase as the inverse of the operating frequency. Size, weight and cost considerations therefore limit the lower usable frequency of existing transducers to 200 Hz or so. Also the device becomes increasingly resonant as frequency decreases thereby limiting the frequency range over which it can be used.
U.S. Pat. No. 3,437,170 discloses acoustic generators which release bubbles underwater from a number of chambers having different volumes for producing waves having predetermined energy spectra. Also U.S. Pat. No. 3,493,072 discloses an acoustic generator for marine seismic exploration in which pressurised gas is released from an underwater chamber impulsively to create air bubbles having varying fundamental frequencies.